In this section

Historical Rules · 11 November 2016

Rule 903.9

Official source text from the 11 November 2016 Rules release

If a commander would be exiled from anywhere or put into its owner’s hand, graveyard, or library from anywhere, its owner may put it into the command zone instead. This replacement effect may apply more than once to the same event. This is an exception to rule 614.5.

Find this number in the current Rules